1. Field of the Invention
This invention relates to an improved process for recovery of hydrocarbons from a porous formation or reservoir. More particularly, the invention relates to an improved enhanced oil recovery process for recovery of hydrocarbons from a porous reservoir which involves injecting a surfactant solution and a gaseous fluid into the reservoir.
2. Background
A large portion of the original oil in place in many oil-bearing subterranean formations remains in place after primary production and waterflooding. As oil reserves dwindle and exploration for new discoveries becomes more difficult and costly, the use of enhanced oil recovery techniques on previously discovered resources will play an increasingly important role in the overall production of crude petroleum.
One enhanced oil recovery (EOR) technique involves injection into the formation of slugs of a micellar fluid followed by a polymer drive fluid. The micellar fluids are aqueous solutions which contain surfactants at relatively high concentrations above the critical micelle concentration. Polymers, such as polysaccharides and partially hydrolyzed polyacrylamides, develop aqueous phase viscosities that provide stable displacement through the formation with reduced bypassing of the surfactant fluid bank. Field experience with the technique has been disappointing under some reservoir conditions. Poor performance of these fluids can be traced back to the fact that, because of the cost of these systems, the micellar/polymer fluids can be injected only in slugs, rather than continuously, if economic recovery of crude oil is to be achieved. The slug size is generally limited to less than about 20% of the reservoir pore volume, for example. Also, the integrity of the slug is weakened by numerous factors, including high formation temperatures (e.g., greater than about 200.degree. F.) and shear forces which have a deleterious effect on the polymers currently being used.
Another concern in using the micellar/polymer fluids is driving fluid mobility. Often a driving fluid, e.g., water, is used to drive the micellar/polymer fluids through the reservoir or formation. Care must be exercised to avoid the driving fluid by-passing the micellar fluid bank, sometimes referred to in the art as "fingering". The driving fluid should push the micellar fluid through the formation toward the production well or wells. Mobility control of the driving fluid provides for effective "pushing" action and reduces by-passing. Such mobility control has been difficult to achieve, with the result that the driving fluid often "breaks through" the slugs of micellar fluid as it sweeps through the formation toward the production well or wells. U.S. Pat. No. 3,648,772 to R. C. Earlougher, for example, uses a combination of foam and a liquid mobility buffer slug to provide mobility control for a micellar dispersion, including a hydrocarbon and a surfactant in an aqueous medium.